feat: adds keys-only filtering (#51)

* feat(bigtable/cmd/cbt): Add keys-only option to lookup and read to avoid fetching and displaying cell values (#4286)

* Refactor a little to make functional testing practical.

Note that tableLike is likely to grow as we test more. :)

* testing helpers

* refactored stdio capture to be in-memory

* Added some assertion functions

* Added getDataFilter and started wiring it into doLookup and doRead

* Finished wireing up getDataFilter to doLookup and doRead

Also, getDataFilter now takes extra filters as optional input,
which is tested in testing doRead.

* Added creds to config to avoid spewing to stderr

* gofmt

* Added missing licence header

* rearranged comments -- build please :)

* Simplify assertNoError using t.Fatalf. doh!

* Refactored (de-refactored :) ) to make diff smaller

* Revert "Refactored (de-refactored :) ) to make diff smaller"

This reverts commit 09164356daaf4913db8c6c3bd7556a7bada65423.

Because there was a reason to factor out filter logic. :)

* Got rid of nested if

By using the original construct.

* simplified expression.

No need for apend because the result is know to have one element.

Co-authored-by: Christopher Wilcox <crwilcox@google.com>
Co-authored-by: Eric Schmidt <erschmid@google.com>

* chore: cleaning up PR

* chore: cleanup

* chore: clean up

* chore: cleanup

* chore: cleanup

* chore: cleanup

* chore: clean up

* chore: cleanup

* chore: cleanup

* test: fix

* fix: per reviewer

* fix: per reviewer

* fix: tests

Co-authored-by: Jim Fulton <jim@jimfulton.info>
Co-authored-by: Christopher Wilcox <crwilcox@google.com>